We have detected a naloxone-like material in the venom of polistes annularis. Upon purification, the active material was found to be polisteskinin, an octadecapeptide which contains the bradykinin sequence at the C-terminus. This led us to investigate the rest of the known peptides of the kinin family. We found that bradykinin is the most potent peptide of the series, with an ED-50 in the picrmolar range for the reversal of the effect of morphine on the longitudinal muscle-myenteric plexus preparation.